Fabrication and characterization of composites comprising (CHA)SAPO-34 with (MFI)ZSM-5 topologies and their catalytic performances on MTO reaction

Abstract

The promising composites comprising of (CHA)SAPO-34 with (MFI)ZSM-5 structures were successfully fabricated by microwave-assisted hydrothermal synthesis and characterized to further investigate their catalytic performance on methanol to olefins (MTO) reaction. The physical mixture, synthesized nanosized ZSM-5 and microsized SAPO-34 were employed as references for exploring the structure-activity correlation. These obtained products were extensively characterized by XRD, SEM, FT-IR, NH3-TPD and N2 adsorption-desorption techniques to investigate their crystalline phase, framework, morphology, acidic sites and pore structure. The SAPO-34/ZSM-5 and ZSM-5/SAPO-34 composites exhibited significantly different frameworks, morphologies, acidic properties, and micro- and mesopores ratios in spite of their similar crystalline phase compositions. The pre-treatment of microscale SAPO-34 impregnated with tetrapropylammonium hydroxide was a determining procedure to obtain well-defined SAPO-34/ZSM-5 composite without phase detachment. The catalytic test results showed that SAPO-34/ZSM-5 composite exhibited excellent catalytic performances with methanol conversion of 98.9% and light olefins selectivity of 91.93% during 60 h continuous reaction at 400 °C as well as induction period of 0.5 h. O2-TPO experimental results showed that low- and high-temperature coke deposition coexisted in SAPO-34/ZSM-5 composite even after 60 h by suppression on diffusion restriction. A synergetic effect of interface phase together with predominant weak acid sites and distinct pore structure is assigned to make essential contributions to enhanced catalytic activity to methanol conversion, superior shape selectivity to light olefins and improved stability for SAPO-34/ZSM-5 composites at mild temperatures of MTO reaction.